When a hawkmoth (Manduca sexta) is searching for its next meal it has more than just its sight to go on: dinner has to smell right, too. While moths can locate food sources using sight or smell alone, in reality the situation is more complicated. Moths integrate information from both senses to evaluate which food items are potential tasty morsels. Concentrating on the visual and olfactory cues that attract moths to flowers, Joaquín Goyret and colleagues Poppy Markwell and Robert Raguso at the University of South Carolina investigated how moths use this information to make a decision to feed or not ().
To attract the moths, the team used white artificial flowers as the visual cue, accompanied by a cotton swab soaked in bergamot oil as the attractive olfactory cue. They placed both objects in a wind tunnel, passing a flow of air over them which created an odour plume from the cotton swab for the moths to smell. They found that when the flower and the cotton swab were in the same place, creating the impression of a scented flower, most of the moths responded by approaching the flower and preparing to feed by extending their proboscises.
`Then we started separating the odour source from the flower, to find out how these two stimuli are used when a moth is deciding to extend its proboscis' says Goyret. The team found that as they increased the distance between the flower and the bergamot odour, the moths' responses dropped off dramatically. Of those moths that did prepare to feed, they took longer to make that decision than when the flower and the odour were in the same place.`The animal is evaluating both signals, and the spatial separation is creating a conflict in the decision making process, making them less likely to approach the flower and try to feed,' Goyret says.
Having shown that separating the two cues by distance affected the moths'response, the team wondered what would happen if they were separated in time. They already knew that the moths were not very likely to try and feed when they were using just sight or smell alone. Would presenting a puff of bergamot odour before the moths saw the flower make them more likely to respond to the scentless flower? The answer was yes; `the response was much higher than to a scentless flower without prior olfactory stimulation,' says Goyret; although the response was still not as high as to a scented flower. Finally to test which cue the moths innately preferred, they were given a choice between the flower, or the bergamot odour. The flower was most moths' first choice, but some of them also chose the odour after visiting the flower, showing that while moths prefer visual signals, they rely on smell too.
These results show that the moths' decision making process is very flexible. Not only do they rate cues in order of importance, they integrate information about when and where they perceived them to decide whether to feed or not. `[A moth] is not just a visual or an olfactory animal,' Goyret explains, `maybe we could better understand sensory systems, and how they affect behaviour, by addressing them as sub-systems that work together and have evolved together, instead of as isolated components'.
